1. Field of the Invention
The present invention generally relates to a wheel spin control apparatus for use in an automotive vehicle for minimizing the spinning of a driven wheel and, more particularly, to the control apparatus operable to control both the braking force, delivered to the driven wheel, and the engine torque, thereby to quickly and effectively minimize the spinning of the driven wheel of an automotive vehicle which would occur, during, for example, the start or acceleration of the automotive vehicle.
2. Description of the Prior Art
An automotive vehicle usually has at least one pair of driven wheels which are coupled together through a differential gear assembly which is in turn coupled with an engine. It has often been experienced that, when a driver starts or accelerates the automotive vehicle, for example, with a full throttle open, an extremely high engine torque is delivered to the driven wheels so that the drive forces applied to the driven wheels will be greater than the frictional forces between the tires on the driven wheels and the road surface. Accordingly, the driven wheels slip or spin excessively relative to the road surface. An efficient and effective transfer of the wheel traction from the tires to the road surface can be achieved when the speeds of the driven wheels' rotations slightly exceed the vehicle speed with a small amount of spin occurring between the driven wheel tire and the road surface. Thus, the excessive spin results in a loss of the engine power and a reduction of driving efficienty. This is also true even in the case where the automotive vehicle is driven at a moderate engine torque, but starts on a slippery road surface.
As such, numerous wheel spin control apparatuses have previously been suggested for relieving the excessive slip to a value required to achieve the maximum traction, that is, the maximized transfer of a tractive force from the driven wheels onto the road surface. For example, some are designed to apply a braking force to the driven wheel and others are designed to reduce the engine torque in the event that the detection of the wheel speeds indicates incipient spin conditions.
A combined version of these two types is disclosed, for example, in Japanese Laid-open Patent Publication No. 58-16948, laid open to public inspection on Jan. 31, 1983. According to this publication, the wheel spin control apparatus is selectively operable in two modes; the braking control mode and the torque control mode. The braking control mode is brought into effect only when one of the left-hand and right-hand driven wheels tends to spin, so that the braking force can be applied to such one of the driven wheels, thereby to substantially eliminate an unbalanced condition of the driven wheels. On the other hand, the torque control mode is brought into effect when both the left-hand and right-hand driven wheels tend to spin and, also, when one of the driven wheels tends to sping during a high speed driving, so that the engine torque can be reduced. Thus, according to the above described prior art, the braking control mode and the torque control mode are independently performed based on the behavior of the driven wheels.
It is generally known that the response to a control of the braking force is remarkably faster than that of the engine torque. Accordingly, it is desired that the control of the wheel spin by the application of the braking force to the driven wheels is to be effected not only for minimizing the unbalanced condition of the driven wheels, but also for a case when both the left-hand and right-hand driven wheels tend to spin. In the latter case, the braking force should be applied for a length of time required for reducing the torque of the engine, in response to the control under the torque control mode, to a level low enough to alleviate the excessive wheel spin.
However, the control of the wheel spin by the application of the braking force results in the consumption of extra energy for forcibly suppressing the engine. If the system is so arranged as to release the braking control mode as early as possible through a gradual change from the braking control mode to the torque control mode such that the braking control mode is superseded by the torque control mode, the excessive wheel spin could be reduced with less engine power loss. According to the above described prior art, however, the braking control mode and the torque control mode are carried out independently of each other and, therefore, it has been extremely difficult to establish a control system by which the excessive wheel spin can be obviated in dependence on both the braking force and the engine turque.